Normal wattmeters will read full scale at rated current and voltage at unity
pf. They will be adequate for locked rotor tests. However, at no load, the
pf may be so low that readings are at the bottom end of the range and aren't
very accurate. A low pf meter is one that would reach ful scale at rated
voltage and current at a pf of , for example, 0.5.
This is more of a problem with transformers than with motors and simply may
be a carryover of a practice used for transformers (open circuit tests).
Don Kelly email@example.com
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